LNK2019 Error under Visual Studio 2010

Question

I have created a sample c++ project under Visual Studio 2010 with following files.

A.h

#ifndef A_H
#define A_H

#include <iostream>

void foo();

#endif

A.cpp

#include "A.h"

void foo()
{
    int a = 1;
}

main.cpp

#include "A.h"

int main(int argc, char* argv[])
{
    foo();    

    return 0;
}

I am getting the following output after build:

1>------ Build started: Project: opengl_test, Configuration: Debug Win32 ------

1> main.cpp

1> A.h

1> A.cpp

1> Generating Code...

1>Debug\A.obj : warning LNK4042: object specified more than once; extras ignored

1>main.obj : error LNK2019: unresolved external symbol "void __cdecl foo(void)" (?foo@@YAXXZ) referenced in function _main

1>C:\Users\alp\Projects\Test Samples\opengl_test\Debug\opengl_test.exe : fatal error LNK1120: 1 unresolved externals

========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

What is the reason for this error?

possible duplicate of Visual Studio 2010's strange "warning LNK4042" — Cody Gray
For what it's worth, I cannot repro this in a new, empty project in VS 2010. I just added your 3 code files (A.h, A.cpp, and main.cpp) and then compiled. Works fine, no errors. So the question is, what's different about your setup than mine (and what you've described in the question)? — Cody Gray
@CodyGray I created a new solution and added these files to the project and now builds without any errors or warnings. The thing is the project was first referencing opengl/glut functions and at some point visual studio started giving me this LNK2019 error and it wasn't giving it all the time as I described here stackoverflow.com/questions/8996385/… . Then it started giving it always. So I converted the project into this simple form and it kept giving the error. I don't know what caused the configuration difference between the 2 projects. — Alp Hancıoğlu
So perhaps this question is relevant. Does the OpenGL libraries by chance contain a file with the name A that could be messing the linker up if/when it flattens the hierarchy? — Cody Gray
In that question poster has a directory structure for the files. In my project I didn't have any directory structure, every file was in the same folder with the .vcxproj file. I tried that solution but it didn't resolve the warning. The file's name wasn't A before(it was Listing2_2 which I am pretty confident don't exist in opengl) and I changed it to A to make it simple for the post — Alp Hancıoğlu

Mark Ransom Mark Ransom · Accepted Answer · 2012-01-25T22:49:25

I think what happened is that A.h was in the Source group of the project rather than the Header group, so it was compiled as if it were a .cpp. Since both A.cpp and A.h will generate an object file A.obj, the last one to compile is the only one that got linked. I believe the last one compiled was A.h, which didn't have an implementation of foo(), thus the linker couldn't find it.

LNK2019 Error under Visual Studio 2010

4 Answers